The invention relates to a device for setting the torque transmitted by a friction clutch, having an actuator chain which comprises an electric motor, a step-down gear and a mechanism for converting the rotational movement into a displacement of a pressure plate of the clutch. Friction clutches having a torque which can be set and transmitted are used, for example, in drive trains of motor vehicles, in particular all-wheel drive motor vehicles, as a differential lock or for switching on an axle drive.
This means not only controlled engagement of the friction clutch but also prolonged operation with the torque set precisely, in accordance with the torque which is to be transmitted. In this case, the torque which is transmitted can be varied continuously by means of a regulating or control device in accordance with dynamic driving requirements. High torques to be transmitted require considerable pressure forces and therefore actuating forces, increased further by the presence of a resetting spring which acts in the opening direction.
In order to produce high and proportionable actuating forces, the torque of the electric motor is intensified by a step-down gear and converted by means of a mechanism into an axial actuating force which acts on the friction clutch. The mechanisms are ramp rings with or without balls, cam disks or other mechanisms.
Friction clutches of the generic type having ramp rings are known, for example from WO 91/12152; WO 01/59331; U.S. Pat. No. 4,976,347 and EP-A-368140. In all of these, the axial force required for transmitting the desired torque is set by activating the electric motor in accordance with one of the customary power control methods (current and/or voltage control, pulse-width modulation, etc.). The interrelationship between required motor current and desired torque of the friction clutch is gathered for this from a characteristic curve. Use is often also made of two different characteristic curves, one when increasing, the other when reducing the torque, in order to take account of the friction hysteresis.
In EP-A-368140, a worm gear affected by friction is used as the step-down gear, and an alternating load is produced in order to overcome the effects of friction hysteresis and in order to reduce the frictional forces in the mechanism. In WO 91/12152 and also in U.S. Pat. No. 4,976,347, spur gears which are low in friction and relatively stiff transmission mechanisms are used as the step-down gear.
The disadvantage of these control methods is that they can only be used if the step-down gear and the mechanism have a low friction hysteresis. Use is therefore made of spur gears, but their step-down is limited. In particular, the characteristic curve for dissipating force has to be clearly situated in the positive region to enable precise setting to be possible at all. Furthermore, the setting accuracy is impaired by the fact that the interrelationship between the torque of the electric motor and the torque transmitted changes, in particular because of:                different frictional conditions in the actuating chain (temperature fluctuations, run-in effects, transition between stiction and sliding friction, and        different force of the clutch resetting spring (manufacturing tolerances, clutch clearance).        
The interrelationship between the torque of the electric motor and the motor current is also affected by relatively great inaccuracies:                manufacturing tolerances,        weakening of field as the temperature of the electric motor rises,        irregularities of the torque over the angle of rotation.        
For these reasons, in the case of the known systems, the entire actuator chain, from the electric motor up to and including the mechanism, has to be optimized in respect of minimal friction hysteresis, this generally requiring a small step-down and a relatively large motor, but in any case causing additional costs and also having the further disadvantage that, in order to maintain a high transmitted clutch torque, a high motor current always has to be flowing. Since this operational state normally results in a thermal overloading of the motor, an electromagnetically actuated motor brake is necessary, this causing further costs and having a negative effect on the dynamic behavior of the actuator chain, since the brake always has to be released before the adjustment of the torque begins. Although a brake is not required if a step-down gear having a self-locking worm is used, the characteristic curve of said worm for the dissipation of force is situated to such a deep extent in the negative region that it is not suitable for this reason.
It is therefore an object of the invention, avoiding the abovementioned disadvantages, to enable the torque transmitted by a friction clutch to be set precisely and rapidly, even given high friction in the actuator chain, and given fluctuating friction and also such that it is not impaired by other interfering influences.